Step 1

Season ribs with salt. Let sit at room temperature 1 hour, or chill, uncovered, up to 12 hours. If chilling, let sit at room temperature 1 hour before cooking.

Step 2

Meanwhile, place chiles and 5 cups hot water in a blender and let sit while you prep onion and garlic.

Step 3

Heat broiler. Broil onion and garlic on a rimmed baking sheet, undisturbed, until charred on top (don’t worry if they get super dark), 8–10 minutes.

Step 4

Pop garlic out of their skins and place garlic, onion, red pepper flakes, coriander, cumin, and cinnamon in blender with chiles. Blend until smooth but still speckled with chiles; season purée lightly with salt. Set aside.

Step 5

Heat 2 Tbsp. oil in a medium heavy pot over medium-high. Working in 2 batches, cook ribs, turning occasionally and reducing heat if needed, until browned all over, 10–12 minutes per batch. Transfer to a plate.

Step 6

Carefully pour off oil and wipe out pot. Pour remaining 1 Tbsp. oil into pot and place back over medium-high heat. Add tomato paste and cook, stirring often, until tomato paste begins to separate and stick to pot, about 3 minutes. Stir in chile purée. Return ribs to pot and bring liquid to a simmer. Partially cover pot and cook, reducing heat to low to maintain a very gentle simmer, turning ribs occasionally, and skimming any excess fat from surface, until meat is very tender, 3–3½ hours. (Make sure there's a gap between lid and pot so liquid can reduce.)

Step 7

Add squash to pot and push down so it’s mostly submerged. Bring to a simmer and cook, uncovered, until squash is tender and liquid is thickened, 18–20 minutes. If squash and meat become tender but liquid isn't thickened, transfer squash and meat to a plate and continue to cook until liquid has reached the desired saucy consistency.

Step 8

Serve ribs and squash, topped with yogurt and cilantro, with lime wedges.

Step 9

Do Ahead: Ribs (without toppings) can be made 3 days ahead. Cover and chill.